This invention relates to a filter device for filtering a gas flow, especially for filtering the gas produced by a preferably round or tubular gas generator for an impact protection cushion of a vehicle, with a filter element through which the gas can flow and which exhibits several layers of filter material of differing permeability following one another in the flow direction.
Impact protection cushions for protecting the occupants of automotive vehicles (such collision protection devices being also called "airbag" systems) are filled, in case of need, by pressurized gases produced during the combustion of a gas-releasing charge in a gas generator. In order to retain slag formed during combustion of the gas-releasing material and to cool the slag as well as the combustion gases, the gases flow through a filter element arranged in the gas generator. The filter element consists of several layers of filter material of differing permeability succeeding one another in the flow direction. Wire mesh of varying mesh widths and fine filter mats of metal fibers are utilized as the filter element and/or filter element plies. In case of round and tubular gas generators, the individual wire mesh plies of the filter element exhibit tubular, cup, ring, or hat shapes of varying diameters. The individual thus-formed wire mesh fabrics or fine filter mats are prefabricated as individual cylinders or shaped articles and then assembled. The manufacture of a filter element having this structure is relatively expensive and thus uneconomical.